They spent the rest of the game trying to fight off the Nationals, who could not avoid losing for the 100th time this season.
Washington forged a 6-6 tie in the sixth on RBI singles by Cristian Guzman and Willie Harris and the score remained deadlocked until Furcal's blast in the eighth.
Washington reliever Ron Villone (4-6) retired the lead-off man in the eighth, but Furcal hit a 1-0 pitch out to right for his ninth homer of the season.
That made a winner out of Ramon Troncoso (5-4). Jonathan Broxton needed 11 pitches to work a perfect ninth for his 36th save.